Linux Useful System Admin Commands

uptime

This command displays a one-line summary showing...
Example: uptime
11:42pm up 18 days, 8:45, 1 user, load average: 0.00, 0.00, 0.00

• Current time (11.42pm)
• Number of days since last boot (up 18 days)
• Number of users logged in (1 user)
• System utilization for the last 1, 5 and 15-minute intervals (load average

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who

This command will display...

• All users logged in
• tty device(line) they are using
• When they logged in
• Idle time
• Where they logged in from

Options:

• -i display idle time
• -H print column headings

Example: who -iH

USER	LINE	LOGIN-TIME	IDLE	FROM
roger	ttyp0	Jun 5 23:27	.	(omni)

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id

This command displays a one-line summary for the current user listing...

• username and number
• primary group and number
• other groups and numbers

Example: id
uid=1000(roger) gid=100(users) groups=100(users),16(dialout),40(game)

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free

This command displays statistics on memory usage. The values represent KB's.
Example...

	total	used	free	shared	buffers	cached
Mem:	63208	61484	1724	27848	1060	49360
-/+ buffers/cache:		11064	52144
Swap:	128516	6500	122016

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du

This command stands for disk usage and will print the size of the current directory and subdirectories in 1 KB sizes.

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df

This command displays information for each file system.
Example: df

Filesystem	1024-blocks	Used	Available	Capacity	Mounted on
/dev/sda1	497667	301229	170736	64%	/
/dev/sda2	7469935	5721885	1360884	81%	/usr

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env

This command lists all shell (environment) variables and their contents for the current user.

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ps

This command lists process status. There are many options for this command. Below are the two most common.
Example: ps u  (process status for current user)

USER	PID	%CPU	%MEM	SIZE	RSS	TTY	STAT	START	TIME	COMMAND
roger	418	0.0	1.7	1808	1124	p0	S	23:27	0:01	-bash
roger	690	0.0	0.5	872	356	p0	R	00:09	0:00	ps u

• PID = Unique Process ID
• SIZE = Virtual image size; size of text+data+stack
• RSS = Resident set size; kilobytes of program in memory
• TTY = Controlling tty
• STAT = Information about the status of the process
• R = runnable
• S = sleeping
• D = uninterruptible sleep
• T = stopped or traced
• Z = zombie process
• Second field = W if the process has no resident pages.
• Third field = N if the process has a positive nice value

Example: ps ax (process status of all processes for all users and system)

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top

This command displays a full screen of information of the top processes. It will update automatically until the program is stopped. To stop the display, enter q.

12:26am	up 18 days			9:29	1 user		load average: 0.00, 0.00, 0.00
39 processes: 38 sleeping, 1 running, 0 zombie, 0 stopped
CPU states: 0.5% user, 0.0% system, 0.0% nice, 99.5% idle
Mem: 63208K av, 60848K used, 2360K free, 271
Swap: 128516K av, 6500K used, 122016K free,  48696K cached
PID	USER	PRI	NI	SIZE	RSS	SHARE	STAT	LIB	%CPU	%MEM	TIME	COMMAND
769	roger	19	0	584	584	428	R	0	0.5	0.9	0:00	top
1	root	0	0	256	232	196	S	0	0.0	0.3	0:02	init
2	root	0	0	0	0	0	SW	0	0.0	0.0	0:03	kflushd
3	root	-12	-12	0	0	0	SW<	0	0.0	0.0	0:00	kswapd
31485	root	0	0	288	288	232	S	0	0.0	0.4	0:00	mingetty
428	root	0	0	252	224	196	S	0	0.0	0.3	0:00	mingetty
142	root	0	0	264	208	208	S	0	0.0	0.3	0:00	mingetty
9	root	0	0	224	188	172	S	0	0.0	0.2	0:21	update
61	root	0	0	348	308	256	S	0	0.0	0.4	0:31	syslogd
59	root	0	0	464	412	232	S	0	0.0	0.6	0:00	klogd
130	root	0	0	1192	1092	932	S	0	0.0	1.7	0:00	_upsd
73	root	0	0	836	800	352	S	0	0.0	1.2	0:12	named
87	root	0	0	2484	1756	1612	S	0	0.0	2.7	0:00	httpd
90	at	0	0	288	248	224	S	0	0.0	0.3	0:00	atd
93	root	0	0	324	292	244	S	0	0.0	0.4	0:00	cron
96	root	0	0	308	292	228	S	0	0.0	0.4	0:05	inetd
105	root	0	0	324	292	236	S	0	0.0	0.4	0:00	lpd
31763	root	0	0	412	412	328	S	0	0.0	0.6	0:00	mgetty

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System Info - procinfo

procinfo gathers some system data from the /proc directory and prints it formatted on the screen.

Some of the information displayed is

• Last Boot time
• Load Average
• average number of jobs running
• number of runnable processes
• total number of processes
• PID of the last process run (idem)
• Swap info
• Memory resources
• Number of disks
• IRQ info
• Installed modules (with the -a or -m option)
• File Systems (with the -a or -m option)

To automatically update the displayed info every 30 seconds
procinfo -fn30
To exit, simply press q